A method is disclosed for producing a plurality of thin-film solar cells connected in series, arranged on a common substrate and made up by depositing in sequence one upon the other, a first electrode layer, at least one layer consisting of semiconductive material and a second electrode layer, said layers being of large area and specially structured to form the series connection.
In order to be able to satisfy the application-related demands for voltage ranges from 12 to 15 volts, it is necessary to connect solar cells in series. Compared with the connection of discrete crystalline silicon wafers, thin-film solar cells offer a significant advantage, namely, the possibility of production by depositing large-area films onto substrates by spraying, evaporation deposition and plasma discharge processes, said films then being divided into individual solar cell areas and electrically connected.
The DE-OS 28 39 038 represents an example thereof. This paper relates to methods of the type mentioned in the opening paragraph, in which an area of the substrate is initially completely coated with a film of transparent, electrically conductive material on top of which is added a second film of at least one semiconductive material, whereafter sections of said films are selectively removed to form on the substrate a plurality of regularly spaced photovoltaic cells. Subsequently applied is a large area covering film to serve as a second electrode layer. To permit the necessary series connection, this covering layer is interrupted in the direct neighborhood of those strips at which material has been selectively removed from the first electrode layer and from the semiconductor layers above it.
Known methods of achieving separation between individual areas referred to as "structuring" the layers--include, for example, material removal processes of a thermal, mechanical or chemical nature, or those employing structuring bar or lift-off technology. However, such removal methods have considerable disadvantages. For instance, the formation of condensate on a cool point on the substrate when removing material by thermal means; scattered flaked-off material and fissures in the remaining layer in the case of mechanical removal; or under-cutting due to inhomogeneous chemical attack by the etching agent and failure to totally remove all residues of said agent when removing material by chemical means. If the drying conditions for the lift-off material lie in a critical temperature range for the semiconductor, incomplete removal of the lift-off material can result (e.g., metal oxides or organic material such as caoutchouc).
The disadvantages of the previously mentioned removal processes are particularly serious when material particles measuring several .mu.m to almost 20 to 30 .mu.m (e.g., of the conductive transparent first layer) remain in areas (possibly due to condensed material and thus formation of chemical compounds) in which the semiconductor layer(s) are to be subsequently applied. In the case of an amorphous Si solar cell, for example, said layer(s) is/are not even 1 .mu.m thick. Typical thicknesses of the a-Si semiconductor layer are in the order of 500 to 700 nm, so that the presence of such particles can cause short circuits between the first and second electrode layers.
A special disadvantage of the DE-OS 28 39 038 method (in which, as mentioned, the first electrode layer plus the semiconductive material deposited on it are structured together) can also be seen from the fact that, in the case of a chemical removal process, for instance, reactions occur (primarily of an oxidative nature--air, acids) which lead to undefined layer compositions (stoichiometric modification, SiO.sub.2 formation), and consequently degrade the quality of the semiconductor layer. In a-Si solar cells alkaline solutions (NaOH, KOH) are also used to remote areas of material, by which the quality reduction suffered by the a-Si material through alkaline ions is familiar. As already mentioned, mechanical processes for removing the uppermost semiconductor layer material from the electrode layer below it frequently cause mechanical damage to the latter, so that the contact to be finally made between the first and second electrode layers can prove to be inadequate. Thermal removal, e.g., by laser light, also leads to analogous modifications in the upper surface of the first electrode layer.